J. J. FIELD ON THE RATIO-MICRO-P0LAR180OPE. 223 



cessary to make three entire turns of tlie driving wheels ; and 

 then, if the exact selcnite supplement needed for developing the 

 structure be within the compass of the zero setting, some position 

 must be arrived at in which the details sought appear with the 

 greatest possible distinctness. 



Should not this be the case, the selenite cells are to be ungeared, 

 the plates re-set, with their axes more or less out of coincidence, 

 and the former operation repeated, and so on. 



It will thus be seen that the number of variations this instru- 

 ment is capable of producing — variations that may be so conducted 

 as to create a gradually increasing or diminishing effect in the 

 direction that appears to he needed ; and which variations, starting 

 as they do from known data, and proceeding in known ratio, can 

 always be reproduced at will — are almost endless. The ratio move- 

 ment, by spreading out the depolarizing axes of the selenites in a 

 pre-determined order (something after the fashion of the opening 

 of a fan), enables the observer rapidly to aiTive at an approxima- 

 tion to the most perfect optical conditions for viewing any particu- 

 lar structure ; and then, in order to arrive at absolute perfection in 

 the development of the details, nothing is needed but a little time 

 and patience, to change the setting, tooth by tooth (in the direction 

 indicated by the previous adjustments), until further change be- 

 comes detrimental. 



By a very simple notation, fine positions can be instantly recorded 

 and afterwards read at a glance ; and although many trials are 

 generally needed to arrive at the finest effects; still, when any ad- 

 justment giving superlative results with any special object is found, 

 such adjustment will prove by no means to be limited to the single 

 object viewed, but also to embrace somewhere tvithin the limits of a 

 half rotation of the lower prism (the selenites themselves now re- 

 maining stationary) the finest optical development of many other 

 slides containing tissues of the same, or closely allied character. 



For example, I found a magnificent setting for exhibiting the 

 cuticle of the Equisetum ; and I have no vegetable cuticle in my 

 possession that does not come out superbly under the same selenite 

 adjustment, but with a different position of the polarizer. 



So in relation to deep coloured entomological objects difficult to 

 polarize, and many others ; they class themselves under certain opti- 

 cal heads as to the settings, only needing a changed position of the 

 polarizer ; and thus a great amount of time and labour is saved ; for 



